1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic process or the like.
2. Description of the Related Art
Heretofore, image forming apparatuses such as copiers and printers that use an image forming process such as an electrophotographic process, an electrostatic recording process or a magnetic recording process, for example, have been provided with a fixing apparatus for using heat to fuse and fix an unfixed toner image on printing material. As for fixing apparatuses for printing material, in terms of method and configuration, apparatuses that employ a heat roller method, a heat plate method, a heat chamber method and a film heating method are known. Fixing apparatuses all incorporate a heating body, and temperature control is managed by controlling power supply to the heating body, such that the temperature of the heating body is maintained at a prescribed temperature (prescribed image fixing temperature, etc.). Among the various types of fixing apparatuses, film-heating type fixing apparatuses particularly enable power savings and wait time reductions (quick start), since they are able to use a thin film or a heating body with a short warm-up period and a low heat capacity. Also in recent years, a fixing apparatus configured to reduce uneven fusing of toner due to the contours of the printing material by providing an elastic layer in the heating film has been proposed.
With a fixing apparatus employing the film heating method, A/D conversion is performed on the detected temperature output of a thermistor provided on the heating body, and a CPU imports the A/D converted output and compares the detected temperature with a target temperature. The CPU controls power supply to the heating body by PID control that is based on a predetermined control table, according to the comparison result. Power supply to the heating body is controlled by performing ON/OFF control of an AC input voltage using a gate-controlled semiconductor switch (hereinafter, triac), with wave number control or phase control being used for power supply control. Wave number control involves performing ON/OFF control in units of half waves such that the AC input voltage is turned ON for a number of waves and turned OFF for a number of waves every prescribed period, with several waves of the AC input voltage being taken as the prescribed period, and is performed by controlling a power supply rate with the ON/OFF duty ratio during the prescribed period. On the other hand, phase control is performed by controlling the phase angle of individual waves of the AC input voltage. Wave number control characteristically results in low harmonic current but high flicker noise, since the power supply rate is controlled every prescribed period of several waves. On the other hand, phase control characteristically results in low flicker noise but high harmonic current, since the power supply rate can be finely controlled within individual half waves. Accordingly, the respective power supply controls are selected according to the requirements of the fixing apparatus, and in the case of using a 200 V commercial power supply, wave number control rather than phase control is often employed particularly in recent years in order to reduce harmonic current. Thus, for example, in Japanese Patent Laid-Open No. 10-333490, a fixing apparatus configured to switch between wave number control and phase control according to the 100 V/200 V AC input voltage has also been proposed. In Japanese Patent Laid-Open No. 2003-123941, a method has also been proposed for combining phase control and wave number control and performing finer control by reducing harmonic current in comparison to when only phase control is used and shortening the update period of the power supply rate in comparison to when only wave number control is used.
Incidentally, when, in the case where wave number control is used with a fixing apparatus employing the film heating method, the power supply is often OFF at the leading edge portion of the printing material because of wave number control resulting in power supply being turned ON/OFF in units of half waves, fixing failure will occur due to the temperature of the heater suddenly dropping. In order to prevent this, Japanese Patent Laid-Open No. 6-118838 proposes a method of increasing power supplied to the leading edge portion of the printing material. Also, with a fixing apparatus employing the film heating method, particularly a fixing apparatus in which the heating film is provided with an elastic layer, the heating state of the printing material will be destabilized due to the printing material entering the heating nip portion. This is caused by the large fluctuation in temperature that occurs in the heating nip portion due to the heating film temperature suddenly dropping because of heat suddenly being absorbed by the printing material when the printing material enters the stable temperature state, and the overshoot that arises when the temperature subsequently increases. In view of this, a method of correcting power supplied to the heating body before the temperature fluctuation due to entry of the printing material occurs is disclosed by the applicant in Japanese Patent Laid-Open No. 2004-078181.
Incidentally, when the temperature of the heating film drops suddenly after the printing material enters the heating nip portion, the temperature of the portion where the heating film has dropped in temperature will still be low when the heating film again contacts the printing material after completing one rotation. As a result, the temperature of the heating film will be low at the portion of the printing material corresponding to the second rotation of the heating film, causing a phenomenon to occur in which the gloss of the image is reduced. On the other hand, the large drop in the temperature of the heating film only occurs immediately after entry of the printing material, and the temperature state is soon stabilized to some extent by PID control, eliminating the temperature drop. Accordingly, even at the portion of the printing material corresponding to the second rotation of the heating film, the reduction in the gloss of the image is only at the portion near the leading edge of the second rotation. However, since the gloss of an image differs greatly between the portion near the leading edge of the second rotation of the heating film and the portion near the trailing edge of the first rotation, the difference in gloss may appear as a clear difference in levels at the boundary therebetween, with this being particularly noticeable when glossy paper is used.
In order to reduce this difference in gloss level, the above-mentioned power correction has to be more finely controlled, such that the gloss is the same where the first and second rotations of the heating film join. That is, even when heat is absorbed and the temperature drops at the leading edge portion of the first rotation, the temperature drop at the leading edge portion of the second rotation of the heating film has to be compensated for, such that the leading edge portion of the second rotation and the trailing edge portion of the first rotation will be the same temperature. Thus, when power correction for forcibly inputting a prescribed amount of power is performed before entry of the printing material, the forcibly input power, or in other words, heat energy, is conveyed to the heating film surface for one rotation, even when the heating film surface experiences a one-off drop in temperature due to entry of the printing material. The temperature then returns to the prescribed temperature when the portion where the temperature dropped is offset and the leading edge portion of the second rotation of the heating film that corresponds to the portion where the printing material initially entered again contacts the printing material. Accordingly, with this control, the timing at which power correction is performed is determined based on the entry timing of the printing material.
As is clear from this mechanism, the inner surface portion of the heating film that is warmed by the heat produced by the power correction has to substantially coincide with the portion where the temperature dropped due to the entry of the printing material, with an even more rigorous accuracy required than in the case where temperature control is merely stabilized. Since the glossiness of printing material such as glossy paper in particular is extremely sensitivity to temperature, and very slight differences in temperature appear as a difference in gloss level, the range within which surface temperature should be controlled is very limited. Further, in order to make the trailing edge portion of the first rotation of the heating film and the leading edge portion of the second rotation the same temperature, power correction for accurately compensating for the temperature drop at the leading edge portion of the second rotation needs to be performed, and thus not only power but also highly accurate timing of when to perform the power correction is required. When the power correction timing deviates even slightly from the proper correction timing, either there will be insufficient power to be able to fully compensate for the temperature drop or a hot offset or the like will occur due to excessive power input, weakening the effect of the power correction. Also, with a fixing apparatus employing wave number control, there is a problem in that temperature fluctuation due to entry of the printing material cannot be adequately reduced because correction cannot be performed at the timing at which power correction should be performed in response to entry of the printing material. This is caused by the fact that since the period for updating the power supply rate of wave number control is in units of half waves, the update period is extended, and, as a result, there are virtually no instances where the update timing coincides with the power correction timing.
FIG. 8 is a timing chart indicating the update period of the power supply rate of wave number control and phase control, and the timing of printing material entry and power correction. In FIG. 8, A indicates the power supply rate update timing of wave number control, with the power supply rate update period of wave number control being 20 half waves. When the power supply frequency of an AC power supply is 50 Hz, the duration of one half wave is 10 msec, and the update period of wave number control is 200 msec. B indicates the power supply rate update timing of phase control, with the update period being two half waves (=20 msec). A power correction start command and a power correction end command are issued such that power correction is started 150 msec before entry of the printing material to the heating nip portion (timing C) and ends 50 msec (timing E) after entry of the printing material to the heating nip portion (timing D). Since wave number control has a long power supply rate update period, the timing at which power correction is actually performed may deviate significantly from the proper power correction timing. For example, since the wave number control shown in FIG. 8 controls the power supply rate in units of 20 half waves (=200 msec), a gap (delay) of 200 msec at most occurs before correction is actually executed after the power correction start command is issued. Since the power correction time period shown in FIG. 8 is 200 msec in total, consisting of 150 msec before and 50 msec after printing material entry, in the case where the power correction start is delayed the most, power correction will be started at the power correction end timing. That is, since the power correction end command will be issued at the same time as the power correction start, power correction will not actually be performed.
In the example described above, since the power supply rate is changed after the correction start command is issued, power correction will be executed later than the power correction start command. Alternatively, although not coinciding with the power supply rate update start timing, if power correction is performed at the power supply rate update timing nearest the power correction start timing, the maximum deviation (delay) in power correction start timing will be slightly alleviated. However, even in this case there will still be a maximum deviation of ±100 msec from the proper power correction timing. FIGS. 9A to 9C are graphs showing the temperature state of the heating film surface in the case where the power correction timing deviates in this manner, with the horizontal axis of each graph showing time and the vertical axis showing surface temperature of the heating film. FIGS. 9A, 9B and 9C respectively show the temperature state of the heating film surface in the case where power correction is performed at the proper timing, power correction is started before the proper timing, and power correction is started after the proper timing. While there is a drop in the temperature of the heating film due to the printing material entering the heating nip portion, in FIG. 9A the difference in the surface temperature of the heating film before and after entry of the printing material to the heating nip portion is kept to about 2° C. In contrast, in FIG. 9B, since there is a significant rise in surface temperature before the printing material enters the heating nip portion, the difference in the surface temperature of the heating film before and after entry of the printing material to the heating nip portion will be 8° C. In FIG. 9C, since there is a significant drop in surface temperature due to the printing material entering the heating nip portion, the difference in the surface temperature of the heating film before and after entry of the printing material to the heating nip portion will similarly be about 8° C.
As shown in FIG. 9B, when power correction is performed before the proper timing, the temperature of the heating nip portion will rise too much and overheat. When the printing material carrying the toner image enters the overheated heating nip portion, the toner becomes excessively fused and hot offset occurs. Also, since a large amount of power is supplied earlier than the proper timing, the temperature of the heating film becomes too high before entry of the printing material, and the gloss of the printing material increases at the trailing edge portion of the first rotation of the film. As a result, horizontal band-like gloss unevenness occurs in which the difference in gloss level between the trailing edge of the first rotation and the leading edge of the second rotation of the heating film is further accentuated. On the other hand, when power correction is performed after the proper timing as shown in FIG. 9C, it will be impossible to compensate for the reduction in heat caused by entry of the printing material, and the temperature of the heating film will drop significantly. In this case, the gloss of the leading edge portion of the second rotation of the heating film will be too low, resulting in gloss unevenness in which the difference in gloss level between the trailing edge portion of the first rotation and the leading edge portion of the second rotation is clearly apparent. To cope with this problem, the update period of the power supply rate can conceivably be shortened, for example, but since the wave number of the update period decreases in this case, the power supply rate cannot be finely set, giving rise to problems with temperature control. Incidentally, even in the case of phase control, deviation in the power correction timing occurs, similarly to wave number control. In the example of FIG. 8, although the amount by which the timing deviates is one full wave (=20 msec) at most, the effect of even with this degree of deviation cannot be considered insignificant. Also, in terms of preventing fixing failure at the leading edge of the printing material, an effect is obtained if the AC input voltage is reliably turned ON at the timing at which the leading edge of the printing material enters the heating nip portion, although in the case where power correction is performed on the printing material, it is important to perform power correction at the prescribed timing and for the prescribed time period. That is, it is important for supply of the prescribed amount of power to be averaged throughout the power correction time period, and thus the power correction time period is set so as basically be an integer multiple of the power supply rate update period.